In one widely used construction of a tennis shoe, the shoe upper is adhered to an insole piece of tough artificial soling material, such as Texon or Bontex, and then a rubber or polyurethane outsole is affixed to the insole and upper, such as by adhesion or stitches. The Texon or Bontex insole pieces are flat, which greatly facilitates the manufacturing of the shoe. But, because the inside of the shoe thus formed is also flat, and made of the relatively hard insole material, manufacturers usually slip a resilient foam insert piece into the shoe which is shaped in the heel and instep areas to cradle the foot and thus make the shoe more comfortable.
When running or walking, these various materials all return to their original flat shapes when the foot is lifted off the ground. When the foot again steps down, the materials compress. But, because the foot is not flat, certain areas of the foot make contact first and press down harder. In shoes with a normal EVA midsole, in fact, the foot is never fully in contact with the shoe.
It would be desirable to have a sole with an upper surface which matches the individual contours of the foot. With such a sole, the impact of running or walking would be taken up more uniformly across the foot area, and the shoe would feel much more comfortable. Also, the foot would slip less inside the shoe.
Since all feet have different shapes, anatomically molded components, such as the insert piece described above, cannot possibly fit everyone. Only a moldable component can conform to any foot and therefore fit properly.
One technique for making a sole contoured to the foot is that which is used in making certain customized ski boots. In this technique, foam is blown into the boot while on the foot of the wearer. Even if this technique could be adapted to tennis shoes, it would be prohibitive in cost.
More recently, in U.S. Pat. No. 3,730,169, it is proposed to modify the slip-in insert piece to have dual layers, one of which is of the normal resilient material and the other of which is a material that permanently deforms to adopt the shape of the foot. The proposal according to the '169 patent thus seeks to improve the function of the insert piece, which as described about is present as a remedy to the fact that the sole itself is flat, rather than anatomically shaped, and that the insole piece makes the sole not particularly resilient. The patent does not address the structure of the shoe itself.